Many metal oxides can be prepared as colloidal solutions or sols. A sol may be defined as a mixture of colloidal particles dispersed in a liquid. In such a mixture, the size of the individual particles are small enough (&lt;1000 .ANG.) to allow for their continuous suspension in solution.
These sols are prepared via hydrolysis of a variety of molecular species. The most well-known example is Si(OC.sub.2 H.sub.5).sub.4 which can be hydrolyzed to form extended Si--O--Si networks. As the hydrolysis process progresses, larger particles are formed as a result of the further branching of these networks. This growth process is then stopped through dilution or addition of charged species thus stabilizing the sol particles in a certain size range. The synthesis of a variety of colloidal systems have been studied in detail, and many of these sols are commercially available.
The formation of uniform spherical particles with small particle diameters are of interest for a variety of applications, especially because of their high surface areas. For example, spherical sol particles of SiO.sub.2 with a diameter of 40 .ANG. have a theoretical surface area of 750 m.sup.2 /g. Once the solution is concentrated, however, these spherical particles aggregate to form larger particles. This description is similar to the concept of Ostwald ripening which states that the larger particles grow larger at the expense of the smaller particles because of the higher solubility of the latter.
Once the particles become very large, gelation of this colloidal mixture takes place. The resulting gel is a solid polymeric network which contains a fluid component. The removal of the excess liquid results in the formation of a highly aggregated 3-dimensional network of metal oxide particles. The aggregation/growth of these particles is intensified upon heating which initiates the sintering process. A significant decrease in surface area is observed as the inter-particle boundaries become less defined and the aggregated network is gradually converted into a glassy phase.
It is obvious that the aggregation and sintering of the original sol particles are not desirable in applications where porous materials with high surface areas are sought. In addition, many applications of such materials as adsorbents or catalysts require elevated temperatures which in turn promote the sintering of the particles. Therefore, there is considerable interest in designing new systems in which these particles are stabilized.
Smectite clays are natural or synthetic layered oxides such as bentonite, montmorillonite, hectorite, fluorohectorite, saponite, beidellite, nontronite, and related analogs. The layers are made up of a central octahedral sheet, usually occupied by aluminum or magnesium, sandwiched between two sheets of tetrahedral silica sites. These negatively charged layers are approximately 10 .ANG. thick, and are separated by hydrated cations such as alkali or alkaline earth metal ions. These cations may be exchanged by other inorganic or organic cations. This feature in addition to the low charge density of the layers allow for the intercalation of a variety of species into the interlayer regions or galleries.
Prior art in the field of pillared clays has shown the formation of new phases in which oxide sols are regularly intercalated in the galleries of smectite clays. For example, see Lewis and Van Santen, U.S. Pat. No. 4,637,992; Yamanaka et al., Mater. Chem. Phys. 17, 87 (1987); Occelli, Proceedings of the Int. Clay Conf., Denver, 319 (1985). The resulting compositions no longer exhibit the X-ray diffraction properties of the parent clay. Instead, the basal spacings of the resulting pillared products are greatly enhanced due to the expansion of the clay galleries by the metal oxide particles. These pillared clay systems prepared from the sols have somewhat larger interlayer spacings compared to the traditional pillared clays (for example, see Vaughan et al., U.S. Pat. Nos. 4,248,739, 4,271,043, and 4,666,877; Pinnavaia et al., U.S. Pat. Nos. 4,665,044 and 4,665,045) formed using smaller molecular units. The pillaring phenomenon allows the basal surfaces of the host clay to be accessed by guest molecules for adsorption and possible catalysis.
Furthermore, it has been shown, see Pinnavaia and Johnson, U.S. Pat. No. 4,621,070, that a tubular aluminosilicate known as imogolite can be intercalated between the layers of montmorillonite. The imogolite tubes are approximately 23 .ANG. in diameter and &gt;2000 .ANG. in length. This particular example represents one of the largest sols which has been directly intercalated in the clay galleries. This composition, as well as all of the other phases which have been reported, require high clay:sol ratios to form a regularly intercalated system.
The purpose of this invention was to inhibit the growth and sintering of finely divided metal oxide sol particles during the drying and dehydration process. This was accomplished by segregating the sol particles with smectite clay particles so that their fusion and further growth was impeded. Sol segregation can be achieved without the need for regular intercalation and pillaring of the host clay. Moreover, the clay can be substantially laminated as in the pristine natural clay such as sodium montmorillonite or it may be substantially delaminated as in the synthetic hectorite clay known as Laponite.RTM.. In either case, the finely divided metal oxide sol particles are entrapped within the composite network over a scale length of microns or less, but greater than the scale length of 10 .ANG. characteristic of pillared clays. It thus becomes possible to heat the composite materials to elevated temperatures without loss in the surface area of the entrapped finely divided metal oxide particles. Binding of the sol particles to the clay surfaces is believed to play an important role in stabilizing the sol particles toward growth and sintering.